1. Field of the Invention
The invention relates to a character printing device and more particularly, to an impact printer for printing with a plurality of impression pressures and for erasing or overstriking misprinted characters.
2. Description of Pertinent Information
Impact-type typing devices such as electronic typewriters can be used to type characters on a set of carbon copy forms 10, as shown in FIG. 1(b). The set is composed of a top form 2, shown in FIG. 1(a), and two sheets of carbon paper 4, 4, positioned between the top form and a middle form 6, and between the middle form and a bottom form 8. The middle and bottom forms 6 and 8 are identical to the top form 2.
Unlike typing on a single form, care must be taken by the operator when performing printing or correcting on a set of carbon copy forms. Otherwise, the carbon copies will not be legible, or the correction will not be completely made. This is due to several reasons.
First, the force with which the typing elements must be impressed on a set of carbon forms is greater than the force with which the typing element must be impressed on a single sheet of paper. Therefore, if the operator forgets to check or change the impression force when typing on a set of carbon copy forms, a clear overstrike may not be made on the bottom-most sheet of the set. This situation is shown in FIG. 2(a). Thus, the operator must make sure that the impression force is increased when typing on a carbon copy form in order to leave a clear overstrike on all of the sheets of the set, as shown in FIG. 2(b).
Second, performing the correction operation is complicated. Correcting an incorrectly typed character is typically performed by striking over the incorrectly typed characters with another character, as shown in FIG. 3(a). In this example, numbers "1,2,3,4,5, and 6" are struck over with an "X". However, before this can be done, the typing portion of the typewriter must be backed up to the position of the incorrectly printed characters. In addition, a typing element must strike over the exact number of characters that are incorrectly printed. Any character may be used for such an overstriking operation, as shown in FIG. 3(b), which shows overstriking of an "X", an equal sign, and a Roman Numeral. However, the operator must usually use the same character for the overstrike operation as the printed character to be erased in order to ensure complete erasure.
Consequently, performing printing and correcting operations on sets of carbon copy forms is complicated for the operator and requires a complex typewriter.